The present invention relates to an improvement in electric vehicles driven by electric motors and, more particularly, to starting control at hill starts.
An electric vehicle of this kind is disclosed, for example, in Japanese Patent Laid-Open Publication No. HEI-3-98404 entitled xe2x80x9cCOMPACT ELECTRIC VEHICLE.xe2x80x9d This electric vehicle includes an electric motor as a driving source, an electromagnetic brake which releases the brake when being energized, a current detecting circuit for detecting the energized state of the electromagnetic brake and a controller which outputs an operation command signal to the electric motor when the current detecting circuit detects the energized state of the electromagnetic brake. That is, upon verification of energization of the electromagnetic brake, the electromagnetic brake is assumed to be released from the braking state and the electric motor is operated. This prevents overload to the electric motor due to so-called brake drag caused by the operation of the electric motor before the elimination of the braking of the electromagnetic brake.
At so-called hill start where an electric vehicle a under electromagnetic braking is started on an upgrade, the conventional electric vehicle rolls back during the time between brake release and sufficient power generation of the electric motor, reducing the riding comfort.
An attempt ot release the brake simultaneously with energization of the electric motor so as to avoid the above problem is not preferable because it would cause the electric motor to start rotating before the completion of the brake release, causing the above-described brake drag, and damaging the brake and the electric motor. An electric vehicle that is capable of effectively preventing brake drag and on-grade rollback is thus desired.
According to the present invention, there is provided an electric vehicle which comprises÷a directional speed member for issuing forward travel, neutral state and reverse travel instructions of the vehicle, a pair of left and right electric motors which operate based on the operation of the directional speed member, a pair of left and right parking electromagnetic brakes for applying holding brakes to the vehicle in the neutral state, and a controller for controlling the start of the vehicle to prevent drag of the parking electromagnetic brakes.
The holding brakes do not mean brakes during travel but brakes such as parking brakes during no travel. Holding braking torque is substantially lower than stopping braking torque. The electromagnetic brakes for holding brakes can be small in volume, leading to reductions in size, weight and cost of the electromagnetic brakes.
The present invention allows effective prevention of brake drag and on-grade rollback of the electric vehicle.
The controller preferably outputs a starting control signal to induce the start of the pair of left and right electric motors with the start of release of the pair of left and right electromagnetic brakes based on information that the directional speed member is switched from neutral to forward travel or reverse travel, and performs such control as gradually reducing the starting control signal output until the electric motors are actually in a rotating state.
The gradual reduction of the starting control signal output means continuous reduction of the signal output relative to time. The gradually reducing speed is set smaller than the releasing speed of the electromagnetic brakes. Too high gradually reducing speed causes too small output of the electric motors before the release of the electromagnetic brakes.
Inducing the start of the electric motors with the start of release of the electromagnetic brakes causes an excess in output of the electric motors due to the small volume of the electromagnetic brakes, causing development of brake drag due to the excessive rotation of the electric motors during the release of the electromagnetic brakes, and shortening the life of the brake shoes. Downward adjustment to gradually reduce the starting control signal output reduces the outputs of the electric motors. This results in practical elimination of the brake drag during the release of the electromagnetic brakes. During the release, the braking torques of the electromagnetic brakes become smaller with time and the electric motors start rotating when the outputs of the electric motors exceed the braking torques of the electromagnetic brakes. Thereafter the operation shifts to normal control.
At hill starts, since the present invention generates a starting control signal for inducing the start of the electric motors with the start of release of the electromagnetic brakes, the sum total of the braking torques of the electromagnetic brakes and the output torques of the electric motors functions as an upgrade rollback stopping force.
The starting control signal output is preferably set smaller than a normal control signal output for rotating the electric motors. If the braking torques of the electromagnetic brakes are too small, the outputs of the electric motors can be excessive, causing brake drag. Setting the electric motor outputs smaller only at starting avoids brake drag. Thus brake drag can be more effectively avoided.
The controller has the functions of: switching the circuits of the pair of left and right electric motors to a short circuit brake mode with the start of release of the pair of left and right electromagnetic brakes based on information that the directional speed member is switched from neutral to forward travel or reverse travel, and maintaining the mode for time t1 required for switching the electromagnetic brakes from a braking state into a released state; switching the circuits of the electric motors to a forward mode or a reverse mode as instructed by the directional speed member while maintaining a control signal to the electric motors at an output corresponding to zero vehicle speed, and maintaining the mode for time t2 required for the circuit switching of the electric motors; and allowing an increase of the control signal output to the electric motors after the lapse of the time t2 and bringing the electric motors into a driving state in accordance with the directional speed member.
Measurements of the time required for the electromagnetic brakes to switch from the braking state to the released state vary because of variation in their mechanical components. A value artificially determined based on an average value of measured values is thus used here as the time t1 corresponding to the time required for the electromagnetic brakes to switch from the braking state to the released state.
As described above, the present invention operates the short circuit brakes while releasing the electromagnetic brakes from the braking state to the released state preparatory to vehicle travel so as to prevent vehicle motion. Then, during the release of the short circuit brakes, the controller outputs a control signal corresponding to zero vehicle speed to cause the electric motors to generate outputs just before travel. This allows the prevention of rollback at hill starts or the like.
After the lapse of the time t1 required for the release of the electromagnetic brakes and the time t2 required for the elimination of the short-circuiting brake circuits, the electric motors are brought into a driving state, so that the occurrence of brake drag can be avoided and increase in volume of the elements used in the electric motor circuits can be limited.